The Swift Burst Alert Telescope (BAT) triggered on an outburst from
the Supergiant Fast X-ray Transient (SFXT) IGR J18410-0535/AX1841.0-0536
on 2011 June 24 at 14:27:04.9 UT (image trigger=455967).
Swift did not slew immediately because the merit value of this new trigger
was less than the merit value of the current target.
The source was detected in a 320-second BAT image trigger (T+0 s) starting
after settling (T-225 s) from a pre-planned slew. Ground processing
reveals a weak detection in BAT (~6.8 sigma, 15-50 keV). The mask-weighted
lightcurve shows some emission (~0.02 counts/cm2/s, 15-50 keV)
in the T-200 to T+0 pre-trigger interval. The emission increases slightly
to ~0.05 counts/cm2/s around T+130 to T+200 s.
A simple power law fit to the spectrum (T+0 to T+320 sec) yields
an index of 2.6 +/- 0.7 (chisq is 63.7, dof=57), and the fluence
is (4.8,-1.9,+2.3)E-7 erg/cm2.
The XRT began a single orbit follow up observation of the field at 15:52:47 UT,
about 5100 s after the BAT trigger. The limited duration of the XRT follow up
(~1 ks) was due to strict temperature constraints.
The source count rate reached a peak at about 7 counts/s and decreased
to about 2 counts/s at the end of the observation.
The spectrum of PC mode data can be fitted (using Cash statistics)
with an absorbed power-law with a photon index of 0.19 (+0.69,-0.30) and an absorbing
column density of NH=[1.5(+2.2,-1.1) ]E22 cm-2.
The average 2-10 keV unabsorbed flux is ~3.6E-10 erg/cm2/s.
This is the second outburst of IGR J18410-0535 detected by the BAT
for which Swift performed a follow up observation that clearly
revealed an on-going flaring activity in the XRT.
Observation results on the previous Swift-detected outburst, which occurred on
2010 June 5, are reported in Romano et al. 2011 (MNRAS,412,L30).
Compared to the 2010 June 5 event, this flare shares consistent spectroscopic
characteristics.
Several on-board BAT detections have been reported (Romano, et al. 2009,MNRAS,399,2021).
The historical light curve from the BAT hard X-ray transient monitor
(Krimm et al, 2006, ATel #904; 15-50 keV) can be found at http://swift.gsfc.nasa.gov/docs/swift/results/transients/weak/IGRJ18410-0535/ .
We would like to thank the Swift Team for making these observations possible,
in particular the duty scientists as well as the science planners.